merge: AgentProject membership — the junction is now load-bearing

Merges feat/agent-project-membership (task 166/152). An agent can finally join
many projects: membership routes, per-membership role and optional brain
override, chat resolving context from the membership, and CLI attach-agent /
detach-agent / get memberships. The legacy Agent.projectId fallback is kept for
this release as the schema comment promises.

Conflict resolved in main.ts: that branch added its RBAC case to
mapUrlToPermission inline, while this branch had extracted the function to
middleware/url-permission.ts so it could be tested. Kept the extraction and
ported the case into it — joining a team is `edit:agents:<name>`, not
`create:agents`, which is the authority to mint a whole agent.

Also fixes something the extraction makes possible. security.test.ts contained a
hand-copied reproduction of mapUrlToPermission, because main.ts self-invokes
main() and nothing in it was importable. A security test asserting against a
copy stops testing the system the moment the real one changes, and this copy had
already drifted. It now imports the real function; all 18 assertions still pass,
against the code that actually runs.

Local: build clean, workspace 2550 passed, lint unchanged at 794 errors.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N4wNHWf7xSwnZCWpJcyv9p

.taskmaster/tasks/task_121.md

@@ -0,0 +1,24 @@
+# Task ID: 121
+
+**Title:** Add AgentSession Prisma model
+
+**Status:** pending
+
+**Dependencies:** 118, 120
+
+**Priority:** high
+
+**Description:** Add AgentSession model with SessionState enum for ephemeral per-session pod tracking
+
+**Details:**
+
+In src/db/prisma/schema.prisma add:
+1. enum SessionState { pending, provisioning, running, ended, failed }
+2. model AgentSession with: id, agentId, projectId, taskId?, containerId?, state, image, expiresAt, lastError? (@db.Text), ownerId, createdAt, updatedAt, endedAt?
+3. Relations: agent (Agent), project (Project), task (Task?), owner (User)
+4. @@index([state]), @@index([taskId]), @@index([agentId])
+5. Add sessions AgentSession[] to Agent, Project, Task, User models
+
+**Test Strategy:**
+
+Schema tests: create AgentSession with all relations, test cascade on Agent/Project delete, test SetNull on Task delete, verify state enum transitions.